Motorway work progresses

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More than 10,000 wick drains have been "planted" beside the causeway on the northwestern motorway to help remove sea water from thick mud as part of the NZTransport Agency's Causeway Upgrade Project.

The drains, made from plastic and fabric, are planted 10 to 15 metres deep in marine mud on both sides of the Traherne Island area.

They work like elaborate drinking straws, placed 1.3m apart to drain water from deep underground to help consolidate the layers of marine materials.

"Intensive ground reclamation like this is an important part of the project's work to upgrade and improve this section of the motorway," highways manager Tommy Parker says.

A 2m to 3m-deep layer of fill, or drainage blanket, using more than 26,000 truckloads of marble-sized quarry stones, has been spread on top of the wick drains to help squeeze water out of the mud below, a process that will take up to 12 months. When the results indicate that the ground is stable, construction of new motorway lanes will start on top of it.

Key features along the 4.8km-long project, being carried out by The Causeway Alliance, include widening the motorway between the Great North Rd interchange and the Whau River bridge at Te Atatu.

When finished in early 2017, the causeway will have been raised 1.5m to stop flooding at extreme high tides.

There will be four lanes city-bound and four/five lanes west-bound with dedicated bus lanes in each direction, and the existing northwestern cycleway between Great North Rd and Te Atatu will be upgraded.